Connecting an External Device to the Beam Detector

Tech Notes

I’m sure you’ve heard ’em before. Walk through the entrance of some convenience stores and you’ll hear that distinctive ding-dong chime alerting the store owner that a customer has just walked in. We do have photo-electric entry systems if you need one, but the topic of this blog is how to connect an external device such as a siren, or maybe even a rotating beacon, to beef up the alerting feature of the entry system.

This idea was actually spawned by a customer who was looking for a system that would alert him if the bucket on his back hoe was lowered enough to allow passage through his garage/workshop door.  I’m guessing that he’s made contact with the top of the opening before.  The intent would be to mount the entry system below the opening such that if the bucket is raised too high, the bucket would break the beam and cause an alert.  Two systems would be required; one for going out of the shop and the second for coming back in.  The entry system has a speaker unit which produces both a chime (ding dong) and an alarm tone (which sounds like a wailing police siren) but the customer suspected that neither tone would be loud enough to overcome the noise of the back hoe.  Which brings us to connecting the extra loud siren.

The entry alert system has an output which is typically used for an optional second speaker or an incremental counter. The output connector is a 3.5 mm stereo jack.  If you want to connect an external device through a relay to drive the siren, get a stereo patch cord with plugs on both ends and use the Tip, positive, and connect it to the plus side of the relay coil.  The Ring, negative, which is the center land, is connected to the minus side of the coil. See attached diagram.  The use of an optional stereo jack or socket to break out the Tip and Ring would simplify the wiring.

When there is no obstruction between the reflector and sensor, the relay will be energized. There will be 12 VDC between the Tip and Ring, so you must use the relay’s COMMON and NORMALLY CLOSED contacts to prevent the externally powered siren from sounding. When there is an obstruction, power to the relay coil is removed, the contact drops, and the siren starts wailing away. As long as there is an obstruction, the relay coil remains unpowered, and the siren continues to sounds. Remove the obstruction, and 12 VDC returns and the siren is silenced.

With regards to monitoring a back hoe bucket, of course all of this is hypothetical. Other factors to consider are the speed of the back hoe, the distance between the entry system and the shop door, and the likely use of a timer to produce a minimum siren duration. Go ahead, play around with it, and just be careful not to take anything out with that bucket.

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